Method of manufacturing hydrated lime

ABSTRACT

A method of providing highly reactive hydrated lime and the resultant lime hydrate where an initial lime feed comprising calcium and impurities is first ground to a particle-size distribution with relatively course particles. Smaller particles are then removed from this ground lime and the smaller particles are hydrated and flash dried to form a hydrated lime, which is then milled to a significantly smaller particle size than that of the relatively course particles.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation of U.S. Utility patent applicationSer. No. 15/938,218, filed Mar. 28, 2018, which is a Continuation ofU.S. Utility patent application Ser. No. 15/596,911, filed May 16, 2017,which is a Continuation of U.S. Utility patent application Ser. No.14/180,128, filed Feb. 13, 2014 and now abandoned, which claims benefitof U.S. Provisional Application Ser. No. 61/772,454 filed Mar. 4, 2013.The entire disclosure of all the above documents is herein incorporatedby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This disclosure is related to the field of quicklime products and themethods of manufacturing thereof, specifically to methods ofmanufacturing compositions comprising calcium hydroxide—more commonlycalled hydrated lime or lime hydrate that has a high purity and highsurface area and is highly reactive.

2. Description of the Related Art

Many efforts have been made to develop materials for improved capabilityof cleaning or “scrubbing” flue gas or combustion exhaust. Most of theinterest in such scrubbing of flue gas is to eliminate particularcompositions, specifically acid gases, that contribute to particularlydetrimental known environmental effects, such as acid rain.

Flue gases are generally very complex chemical mixtures which comprise anumber of different compositions in different percentages depending onthe material being combusted, the type of combustion being performed,impurities present in the combustion process, and specifics of the fluedesign. However, the release of certain chemicals which commonly appearin flue gases into the atmosphere is undesirable, and therefore theirrelease is generally regulated by governments and controlled by thosewho perform the combustion.

Some of the chemicals that are subject to regulation are certain acidgases. A large number of acid gases are desired to be, and are, undercontrolled emission standards in the United States and other countries.This includes compounds such as, but not limited to, hydrogen chloride(HCl), sulfur dioxide (SO₂) and sulfur trioxide (SO₃). Sulfur trioxidecan evidence itself as condensable particulate in the form of sulfuricacid (H₂SO₄). Condensable particulate can also be a regulated emission.Flue gas exhaust mitigation is generally performed by devices called“scrubbers” that introduce chemical compounds into the flue gas thatthen react with the compounds to be removed by either allowing them tobe captured and disposed of, or allowing them to be reacted into a lessharmful compound prior to their exhaust, or both. In addition tocontrolling the emissions for environmental reasons, it is desirable formany combustion plant operators to remove acid gases from their flue gasto prevent the acid gases from forming powerful corroding compoundswhich can damage flues and other equipment.

These acid gases can arise from a number of different combustionmaterials, but are fairly common in fossil fuel combustion (such as oilor coal) due to sulfur being present as a common contaminant in the rawfuel. Most fossil fuels contain some quantity of sulfur. Duringcombustion, sulfur in the fossil fuel can oxidize to form sulfur oxides.A majority of these oxides forms sulfur dioxide (SO₂), but a smallamount of sulfur trioxide (SO₃) is also formed. Selective CatalystReduction (SCR) equipment, commonly installed for the removal ofnitrogen oxides (NO_(x)), will also oxidize a portion of the SO₂ in aflue gas to SO₃. Other components of the process (iron, etc.) canincrease the amount of SO₃ that forms in the flue gas. Particularly incoal combustion, where the chemical properties of the coal are oftenhighly dependent on where it is mined, the ability to mitigate theamount of sulfur oxides in flue gas is highly desirable as it allows forlower quality raw coal (which may be less expensive to produce and moreabundant) to be burned sufficiently cleanly to lessen environmentalimpact and impact on machinery.

SO₂ is a gas that contributes to acid rain and regional haze. Since the1970's, clean air regulations have been designed to reduce emissions ofSO₂ from industrial processes at great benefit to the environment andhuman health. For large emitters, the use of wet and dry scrubbing hasled to the reduction of SO₂. Smaller emitters, however, seek out lesscostly capital investment to control SO₂ emissions in order to remainoperating and produce electricity or steam. Similarly, halides in fossilfuels (Cl and F) are combusted and form their corresponding acid in theflue gas emissions. The halogenated acids also contribute to corrosionof internal equipment or, uncaptured, pollute the air via stackemissions.

Mitigation, however, can be very difficult. Because of the requiredthroughput of a power generation facility, flue gases often move throughthe flue very fast and thus, are present in the area of scrubbers foronly a short period of time. Further, many scrubbing materials oftenpresent their own problems. Specifically, having too much of thescrubbing material could cause problems with the plant's operation fromthe scrubber material clogging other components or building up on movingparts.

Flue gas treatment has become a focus of electric utilities andindustrial operations due to increasingly tighter air quality standards.As companies seek to comply with air quality regulations, the needarises for effective flue gas treatment options. Alkali species based onalkali or alkaline earth metals are common sorbents used to neutralizethe acid components of the flue gas. The most common of these alkalisare sodium, calcium, or magnesium-based. A common method of introductionof the sorbents into the gas stream is to use dry sorbent injections.The sorbents are prepared as a fine or coarse powder and transported andstored at the use site. Dry sorbent injection systems pneumaticallyconvey powdered sorbents to form a fine powder dispersion in the duct.The dry sorbent neutralizes SO₃/H₂SO₄, and protects equipment fromcorrosion while eliminating acid gas emissions. Common sorbents used aresodium (trona or sodium bicarbonate) or calcium (hydrated lime, Ca(OH)₂)based.

One commonly used material for the scrubbing of acid gases is hydratedlime. It has been established that hydrated lime can provide a desirablereaction to act as a mitigation agent. Hydrated lime reacts with SO₃ toform calcium sulfate in accordance with the following equation:

SO₃(g)+Ca(OH)₂(s)->CaSO₄(s)+H₂O(g)

Hydrated lime systems have been proven successful in many full scaleoperations. These systems operate continuously to provide utilitycompanies with a dependable, cost-effective means of acid gas control.

These hydrated lime compositions specifically focus on high surface areabased on the theories of Stephen Brunauer, Paul Hugh Emmett, and EdwardTeller (commonly called the BET theory and discussed in S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309, the entiredisclosure of which is herein incorporated by reference). Thismethodology particularly focuses on the available surface area of asolid for absorbing gases—recognizing that a surface, in suchcircumstances, can be increased by the presence of pores and relatedstructures. The most effective hydrated lime sorbents for dry sorbentinjection have high (>20 m²/g) BET surface area. Two examples of suchcompositions with increased BET surface areas are described in U.S. Pat.Nos. 5,492,685 and 7,744,678, the entire disclosures of which are hereinincorporated by reference. Because of this, commercially availableproducts are currently focused on obtaining lime hydrate withparticularly high BET surface areas. It is generally believed that theBET surface area really needs to be above 20 m²/g to be effective, andin many recent hydrated lime compositions the BET surface area is above30 m²/g in an attempt to continue to improve efficiency. These sorbentsoffer good conveying characteristics and good dispersion in the fluegas, which is necessary for high removal rates. Use of a higher quality,high reactivity source of hydrated lime allows for better stoichiometricratios than previous attempts that utilized lower quality hydrated limeoriginally targeted for other industries such as wastewater treatment,construction, asphalt, etc.

The reaction of hydrated lime with acid gas (such as SO₃) is generallyassumed to follow the diffusion mechanism. The acid gas removal is thediffusion of SO₃ from the bulk gas to the sorbent particles. Thus, highsurface area does not itself warrant a prediction in improved removalsof acid gases. Specifically, high pore volume of large pores isgenerally believed to be required to minimize the pore plugging effectand, therefore, BET surface area has been determined to be a reasonableproxy for effectiveness of lime hydrates in removal of acid gases.Conventional wisdom also indicates that smaller particles act as bettersorbents.

Lime hydrate meeting the above described characteristics, properties,and reactivity has generally been manufactured according to a commonlyknown and utilized process. First, a lime feed of primarily calciumoxide (commonly known as quicklime) is continuously grinded using apulverizing mill until a certain percentage of all the ground particlesmeet a desired size (e.g., 95% or smaller than 100 mesh). In otherwords, all of the lime feed is ground together (lime and impurities),without any removal of particles during the grinding, until the batch oflime feed (both the lime and impurities) meets the desired particle sizerequirements. This continuous grinding is not surprising as theconventional wisdom is that small particles are better and, thus, themore the calcium oxide is grinded the better.

Second, the quicklime meeting the desired size requirements is then fedinto a hydrator, where the calcium oxide reacts with water (also knownas slaking), and then flashed dried to form calcium hydroxide inaccordance with the following equation:

CaO+H₂O→Ca(OH)₂

Finally, the resultant calcium hydroxide (also known as hydrated lime)is then milled and classified until it meets a desired level of finenessand BET surface area.

Despite the wide use of the above described process, there still existmany problems and there remains a desire to produce a more highlyreactive and higher purity hydrated lime. Accordingly, there is a needin the art for a more effective method of manufacturing hydrated lime.

SUMMARY OF THE INVENTION

The following is a summary of the invention, which should provide to thereader a basic understanding of some aspects of the invention. Thissummary is not intended to identify critical elements of the inventionor in any way to delineate the scope of the invention. The sole purposeof this summary is to present in simplified text some aspects of theinvention as a prelude to the more detailed description presented below.

Described herein, among other things, is a method of producing hydratedlime and the resultant lime hydrate where an initial lime feedcomprising calcium and impurities is first ground to a particle-sizedistribution with relatively course particles. Smaller particles arethen removed from this ground lime and the smaller particles arehydrated and flash dried to form a hydrated lime, which is then milledto a significantly smaller particle size than that of the relativelycourse particles.

There is described herein, in an embodiment, a method of producinghydrated lime, the method comprising: providing a lime feed comprisingcalcium oxide and impurities; milling the lime feed to produce a fineground lime, wherein the fine ground lime has a particle-sizedistribution of less than 80% minus 200 mesh; feeding the fine groundlime into an air classification system; removing a refined fine limefrom the air classification system wherein the refined fine lime has aparticle size distribution of greater than 70% minus 200 mesh; addingwater to the refined fine lime to form damp hydrated lime; andflash-drying the damp hydrated lime to form dried hydrated lime.

In an embodiment, the method further comprises, milling the driedhydrated lime to a particle-size distribution of at least 98% minus 325mesh.

In an embodiment of the method, the lime feed comprises no less than 94%calcium oxide and greater than 0% impurities.

In an embodiment of the method, the dried hydrated lime comprisesgreater than 96% calcium hydroxide.

In an embodiment of the method, the dried hydrated lime has a reactivityin citric acid of 10 seconds or less.

In an embodiment of the method, the dried hydrated lime has a BETparticle surface area of greater than 30 m²/g.

In an embodiment of the method, the lime feed comprises about 94%calcium oxide and about 6% impurities.

In an embodiment of the method, the dried hydrated lime comprisesgreater than 96% calcium hydroxide.

In an embodiment of the method, the dried hydrated lime has a reactivityin citric acid of 10 seconds or less.

In an embodiment of the method, the dried hydrated lime has a BETparticle surface area of greater than 30 m²/g.

In an embodiment of the method, the fine ground lime has a particle-sizedistribution of less than 70% minus 200 mesh.

In an embodiment of the method, the fine ground lime has a particle-sizedistribution of between 50% to 60% minus 200 mesh.

In an embodiment of the method, the refined fine lime has a particlesize distribution of greater than 80% minus 200 mesh.

In an embodiment, the method is performed in a low CO₂ environment.

There is also described herein a method of producing hydrated lime andthe resultant hydrated lime produced by the method, the methodcomprising: providing a lime feed having greater than 0% impurities;milling the lime feed to produce a fine ground lime, wherein the fineground lime has a particle-size distribution of less than 80% minus 200mesh; feeding the fine ground lime into an air classification system;removing a refined fine lime from the air classification system whereinthe refined fine lime has a particle size distribution of greater than70% minus 200 mesh; adding water to the refined fine lime to form damphydrated lime; flash-drying the damp hydrated lime to form driedhydrated lime; and milling the dried hydrated lime to a particle-sizedistribution of at least 98% minus 325 mesh; wherein the dried hydratedlime has a BET particle surface area of greater than 30 m²/g.

In an embodiment of the method, the fine ground lime has a particle-sizedistribution of less than 70% minus 200 mesh.

In an embodiment of the method, the fine ground lime has a particle-sizedistribution of between 50% to 60% minus 200 mesh.

In an embodiment of the method, the refined fine lime has a particlesize distribution of greater than 80% minus 200 mesh.

In an embodiment of the method, the method is performed in a low CO₂environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of an embodiment of a method ofmanufacturing hydrated lime.

FIG. 2 depicts a block diagram of an embodiment of a portion of a methodof manufacturing hydrated lime showing an integrated milling andclassification system.

FIG. 3 depicts a block diagram of an embodiment of a portion of a methodof manufacturing hydrated lime showing an integrated milling,classification, and drying system.

FIG. 4 depicts the reactivity of a standard hydrated lime as compared tothe reactivity of hydrated lime produced according to an embodiment of amethod of manufacturing hydrated lime disclosed herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In general, lime feed is subjected to a multi-stage manufacturingprocess (including grinding, classification, hydration, drying, etc.)wherein the resultant product is largely calcium hydroxide. Theresultant hydrated lime product is highly reactive and has a very highpurity and surface area and very small particle size. More specifically,in one embodiment, the hydrated lime has a reactivity in citric acid of10 seconds or less, a concentration of greater than or equal to about96% calcium hydroxide, a particle surface area of greater than 30 m²/g,and a particle size of less than 44 micron for about 98% of theparticles.

It should be noted that values for the reactivity, purity, surface area,and particle size discussed herein are merely exemplary and preferred,and in no way limiting. As would be understood by one of ordinary skillin the art, by altering some or all processing conditions discussedabove or the purity of the initial lime feed, other reactivity, purity,surface area, and particle size values for the final hydrated limeproduct can easily be achieved in accordance with the below describedmanufacturing process.

In order to test reactivity of particular lime hydrate compounds, in anembodiment, the reactivity to a weak acid (such as, but not limited to,citric acid) provides for a reactivity time that is measurable withcommercial instruments. The problem with determining reaction time tostronger acids is that the reaction can be too quick to effectivelymeasure at laboratory scaling. Thus, it is difficult to predictcompositions that will function well without performing large scalepilot testing. In order to determine the citric acid reactivity of aparticular hydrated lime composition, the amount of time it took 1.7grams of lime hydrate to neutralize 26 grams of citric acid wasmeasured. As a measurement of effectiveness, it is preferred that thisvalue be less than or equal to 10 seconds in order to have a hydratedlime composition which is classified as being “high reactive”. Howeverin an alternative embodiment it can be less than 8 seconds. It is morepreferred that this value be 4 or less, 3 or less, 2 or less or 1 orless. Again, given the practical realities that production of improvedmaterial often results in a product having dramatically increased costsof products, utilizing current manufacturing techniques and for currentemissions standards, in an embodiment the lime hydrate may be in the 2-5second range, or, in another embodiment, in the 3-4 second range.

The “lime feed” (also known as “quicklime” and used synonymously herein)that is utilized in the initial stages of the methods disclosed hereinrefers to calcium oxide which is generally made by the thermaldecomposition of limestone (predominately calcium carbonate) in a limekiln. This is accomplished by a process called calcination orlime-burning (by heating the material to an extremely high temperature),which liberates the molecule of carbon dioxide (CO₂), leaving quicklime.The quicklime, however, is not pure calcium oxide and includes otherimpurities (e.g., residual uncalcined stone, dead burned lime, silicanodules, metal oxides, etc.). In some instances, the lime feed may havea chemical purity as low as 94% (i.e., 94% Calcium Oxide (CaO) and 6%impurities). In other instances, the lime feed may have a chemicalpurity less than 94% but greater than 90%, or less than 90%. In oneembodiment, the purity will be about 93%. In still further instances,the lime feed may be greater than 94% CaO. In any event, the lime feeddiscussed herein is not limited to any particular purity of CaO, unlessotherwise noted.

Turning now to FIG. 1, a general block diagram of the method ofmanufacturing hydrated lime will be described in more detail inaccordance with several embodiments. Lime feed (50) is subjected to finegrinding or milling (10) to produce a fine ground lime (51). The fineground lime can be of varying sizes in different embodiments butgenerally has a particle-size distribution of less than 80% minus 200mesh. That is, the lime feed (50) is ground significantly coarser thanthe desired end product. In a preferred embodiment, the fine ground limehas a particle-size distribution of about 50% to about 60% minus 200mesh. Any grinding or milling is suitable, including, for example, finegrind cage mill, swing hammer mill, screen mill, etc. where the amountof milling produces the desired particle-size distribution.

After being milled, the fine ground lime (51), including the impurities,then passes through an air classification system (11). The airclassification system (11) generally comprises an air classifier (e.g.,a standard Raymond or Sturtevant air classifier) or a turbineclassifier. The air classification system (11) generally comprises twosteps, which may be performed consecutively or concurrently.

As noted above, the lime feed (50)—and fine ground lime (51)—generallywill include various different impurities. Without being limited to aparticular particle-size distribution, it is believed that grinding thelime feed (50) to a particle-size distribution of about 50% to about 60%minus 200 mesh grinds a majority of the calcium oxide to the desiredparticle-size distribution (i.e., passing a 200 mesh screen) but doesnot grind the harder impurities to a size below 200 mesh. In otherwords, the harder impurities are larger than 200 mesh while most of thelime particles are smaller than 200 mesh. As a result, removal of theimpurities becomes easier and at an earlier stage of the manufacturingprocess as discussed below.

In the air classification system (11), the harder impurities are thenremoved and a refined fine lime (53) is produced. As the name implies,the refined fine lime (53) will have a smaller particle-sizedistribution than both the fine ground lime (51) and the lime feed (50)as the larger particles from the fine ground lime (51) are removed. In apreferred embodiment, the refined fine lime (53) will have aparticle-size distribution of greater than about 80% minus 200 mesh. Inanother embodiment, it will be greater than 93.5% 200 mesh. In someembodiments, though, the particle-size distribution will be less thanabout 80% minus 200 mesh (e.g., greater than 70% minus 200 mesh, withthe fine ground lime (51) having a particle-size distribution of lessthan 70% minus 200 mesh). The coarse fraction (52) (i.e., the portion ofthe fine ground lime (51) from the air classifier system (11) largerthan 200 mesh and which is believed to include a higher percentage ofthe impurities than the lime feed (50)) is then removed and can be used,for example, as a de-dusted fine granular lime. In some embodiments, thecoarse fraction (52) may be fed into the mill (10) for grinding to thedesired particle-size distribution discussed above and then again fedinto the air classifier system (11). This additional re-grinding of thecoarse fraction (52) can continue as many times as desired or necessary,but is by no means required. In some instances, however, suchre-grinding ensures that the maximum amount of calcium oxide from thelime feed (50) can be utilized in generating the desired hydrated lime,as discussed below.

As can be seen from Table 1 below, the combination of the mill (10) andair classification system (11) results in refined fine lime (53) with ahigher purity (i.e., less impurities and higher concentration of CaO).

TABLE 1 Feed Composition Sample Description CaO CaCO₃ MgO SiO₂ Al₂O₃Fe₂O₃ Sulfur MnO Feed to Mill (50)/Classifier (51) 93.95 1.42 1.0400.800 0.101 0.099 0.024 46 Refined Fine Lime (53) 95.32 0.63 0.825 0.7300.083 0.095 0.026 45

It should also be noted that although the above description refers toparticles passing through a mesh, this description and use of meshmerely refers to the common use of the term mesh as it relates toparticle-size distribution. A mesh screen or sieve need not be used tomeasure or classify the particle-size. Instead, the air classificationsystem described above is used, which generally separates particles lessthan 200 mesh from particles greater than 200 mesh.

Without being limiting to any particular theory of operation, it isbelieved that the removal of potential impurities from the lime feed(50) (and fine ground lime (51)) improves the reactivity of the finalhydrated lime product. As noted above, the impurities are often harderthan the calcium oxide and thus harder to grind. As a result,efficiencies are wasted during this milling step if the lime feed iscontinuously milled until the batch of lime feed meets the desired sizerequirements. Additionally, the continuous milling step of the prior artleads to ultrafine calcium oxide particles. It is believed theseultrafine calcium oxide particles results in a less reactive finalhydrated lime product as the smaller particles results in the formationof more agglomerates, and thus less reactivity of the final hydratedlime product. Finally, the refined fine lime (53) without impuritieshelps resolve a major problem of localized overheating of the limeduring the subsequent hydration that was commonly seen in the prior art.The end result, and the remaining processing steps below, is theproduction of a small average diameter particle and a more reactiveresultant lime hydrate.

As quicklime is generally not stable and, when cooled, willspontaneously react with CO₂ from the air until, after enough time, itis completely converted back to calcium carbonate, all the milling andclassification of the quicklime should preferably be produced entirelyin a closed-circuit system to prevent air slaking and recarbonation(i.e., CaO to CaCO₃) from occurring, although a closed-circuit system isby no means required. In some embodiments, additional measures areemployed to prevent recarbonation. For example, conditioned low CO₂ air(18) can be injected (66) into these systems to replace any air beingdrawn in and around process equipment bearings and seals. Thisconditioned air (18) is also very useful if the quicklime needs to bepneumatically conveyed. The process for conditioning this air isdiscussed more fully below and is also described in U.S. Pat. No.6,200,543 (the entire disclosure of which is incorporated herein byreference).

As noted above, the refined fine lime (53) from the air classificationsystem (11) then undergoes a hydration or slaking process (12). Thehydration process (12) generally comprises a hydrator, and in apreferred embodiment, the refined fine lime (53) is combined with anexcess of water (54) and rapidly mixed which allows the calcium oxide toreact with the water to form damp calcium hydroxide (hydrated lime). Thewater (54) for hydration is generally fed at a reasonably hightemperature but low enough that the refined fine lime (53) is notoverheated (burned). In this regard, the water feed (54) and hydrator(12) temperature should be maintained below the boiling point of water,and more preferably, at a temperature equal to or below 180° F.Utilizing an excess of water (i.e., more than necessary to react withthe CaO) also helps prevent overheating and burning and helps separatethe individual particles. In any event, the damp hydrated lime (56) thatleaves the hydrator (12) should have residual moisture of about 5-35%,and more preferably, about 10-25%.

In addition to the water, other additives (55) may be included in thewater feed (54) that is utilized in the hydration process (53). Theseadditives can help improve the reaction rate of the final hydrated limeproduct. These additives are generally accelerators or retarders, which,as their names suggest, accelerate or retard the reaction of calciumoxide to calcium hydroxide. Any known accelerators or retarders can beutilized, including, alkaline-earth chlorides (e.g., barium chloride,calcium chloride, sodium chloride, potassium chloride, aluminumchloride, etc.), other salts (e.g., aluminum nitrate, sodium carbonate,sodium borate, potassium permanganate, potassium chlorate, table salt,Rochelle salt, etc.), acids (e.g., hydrochloric acid, sulfuric acid,oxalic acid, nitric acid, acetic acid, lactic acid, etc.), alkanols(e.g., mono-, di-, and tri-ethanolamine, dimethylethanolamine, methyldiethanolamine, triisopropanolamine, etc.), and sugars. Further examplesof such accelerators and retarders, and their use in hydrated limeproduction can be found in U.S. Pat. Nos. 1,583,759; 1,649,602;1,664,598; 2,193,391; 2,423,335; 2,437,842; 3,120,440; 4,626,418;4,786,485; 5,173,279; 5,306,475; 5,308,534; 5,332,436; 5,502,021;5,618,508; 5,705,141; 6,322,769; and 7,744,678 (the entire disclosuresof which are incorporated herein by reference).

This damp hydrated lime (56) is then dried (14). In a preferredembodiment, the damp hydrated lime (57) is flash-dried using air (64)from an indirect heat source (17) with a temperature of about 550° F. toabout 850° F. Using indirect heat prevents the hydrate from contactingthe combustion gas which can occur if a direct heat source were to beused. This contact would result in the loss of some of the availablecalcium hydroxide. In any event, the dried hydrated lime (57) generallywill have a residual moisture content of about 1% or less.

As noted above, the presence of CO₂ in air which then comes in contactwith the hydrated lime can compromise the chemical integrity of thehydrated lime. While hydrated lime has greater moisture stability thancalcium oxide, hydrated lime is perishable unless adequately protectedfrom CO₂ absorption and the introduction of CO₂ into the hydrated limecan result in recarbonation (i.e., Ca(OH)₂ to CaCO₃). Thus, in someembodiments, the chemical purity can be further improved if the indirectheater is supplied with conditioned air (63) that has a reduced CO₂content. Examples of apparatuses and methods for such air conditioning(i.e., reduction of CO₂ content in the air stream) are disclosed, forexample, in U.S. Pat. Nos. 5,678,959 and 6,200,543 (the entiredisclosures of which are incorporated herein by reference). In onepreferred embodiment, ambient air (65) (e.g., about 300 ppm CO₂) is fedinto an air conditioner (18), resulting in conditioned air (63) with aCO₂ concentration of less than 100 ppm CO₂. The benefits of this low CO₂conditioned air can be seen in Table 2 below, which shows variouscompositions and BET surface area dried hydrated lime depending on theamount of CO₂ in the air used to dry the damp hydrated lime.

TABLE 2 Effect of CO₂ in Drying Air Available % CO₂ B.E.T. SurfaceCa(OH)₂ in Hydrate % CaCO₃ Area 11-12% 80-84% 6.4-7.2% 15.34% 32-33 m²/gCO₂ (82.54% avg.) (6.75% avg.  (32.57 avg.) 560 ppm 93.5-94.5%   1-1.2%2.29% 35-38 m²/g (94.06% avg.) (1.01% avg.) (37.77 avg.) 120 ppm 94-95%0.6-0.7% 1.41% 37-39 m²/g (94.64% avg.) (0.62% avg.) (38.81 avg.)

As noted above, conditioned air (66) with the same or different CO₂concentrations as the conditioned air (63) for drying can also be fedinto the mill (10), air classification system (11), and/or hydrator (12)to help prevent recarbonation. Additionally, conditioned air (61) can befed into any additional classifiers (15) and/or mills (16), if presentand as discussed more fully below.

After being dried (14), the dried hydrated lime (57) is then classified(15) and milled (16). The dried hydrated lime (57) is first fed into aclassifier (15). If it meets the desired properties (e.g., thosediscussed herein, including purity reactivity, BET surface area, andparticle size), dried hydrated lime (57) is utilized as the finalhydrated lime product (60). Some of the dried hydrated lime (57),however, may not meet the desired properties. This non-final hydratedlime (58) is then fed into the mill (16) to be grinded, with the grindedhydrated lime (59) being fed back into the classifier (15) to determineif the material can be utilized as the final hydrated lime product (60).This process of milling (16) and classifying (15) can continue for aslong as is necessary.

Again, in a preferred embodiment, the milling (15) and classificationsystem (16) are conducted in a closed circuit system to prevent aircarbonation from occurring. Conditioned air (61) (i.e., low CO₂) canfurther be injected into the milling (15) and classification system (16)to replace any transient air being drawn into the process and preventrecarbonation.

The above process of manufacturing describes a process in which thedrying (14), classifying (15), and milling (16) of the damp hydratedlime (56) are conducted independently. As would be understood by one ofordinary skill in the art, milling and classification system can be, andcommonly are, integrated into one system. An example of an integratedmilling and classification system is depicted in FIG. 2, wherein driedhydrated lime (57) is fed into the milling/classification system (40)with injected conditioned air (61) and the resultant final hydrated limeproduct (60) has the desired properties as discussed above. Similarly,an integrated milling and classification system can be furtherintegrated into a dryer. An example of such a fully integrated system isdepicted in FIG. 3, wherein the damp hydrated lime (56) is fed into themilling/classification/dryer system (41) with an indirect heat source(64) and the resultant final hydrated lime product (60) has the desiredproperties as discussed above.

In any event, embodiments of the manufacturing process described hereinresult in a final hydrated lime product (60) with a high purity and highreactivity. In particular embodiments, the final hydrated lime product(60) has a purity of 96% calcium hydroxide or greater, a reactivity incitric acid of less than 10 seconds, a BET surface area of greater than30 m²/g, and a particle size of less than about 44 microns (325 mesh)for about 98% of the particles. As can be seen from FIG. 4, limeproduced by the manufacturing processes described herein has a muchfaster reactivity over lime produced by conventional methods.

While the invention has been disclosed in conjunction with a descriptionof certain embodiments, including those that are currently believed tobe the preferred embodiments, the detailed description is intended to beillustrative and should not be understood to limit the scope of thepresent disclosure. As would be understood by one of ordinary skill inthe art, embodiments other than those described in detail herein areencompassed by the present invention. Modifications and variations ofthe described embodiments may be made without departing from the spiritand scope of the invention.

It will further be understood that any of the ranges, values,properties, or characteristics given for any single component of thepresent disclosure can be used interchangeably with any ranges, values,properties, or characteristics given for any of the other components ofthe disclosure, where compatible, to form an embodiment having definedvalues for each of the components, as given herein throughout. Further,ranges provided for a genus or a category can also be applied to specieswithin the genus or members of the category unless otherwise noted.

1. A method of reducing impurities in a lime feed, the methodcomprising: providing a lime feed comprising calcium oxide and a firstamount of impurities; milling said lime feed to produce a fine groundlime, wherein said fine ground lime has a particle-size distribution ofabout 50% to about 60% minus 200 mesh; feeding said fine ground limeinto an air classification system; and removing a refined fine lime fromsaid air classification system wherein said refined fine lime is finerthan said fine ground lime and has a particle size distribution ofgreater than 70% minus 200 mesh; wherein said refined fine lime includesa second amount of impurities less than said first amount of impurities.2. The method of claim 1, wherein said lime feed comprised more than 6%impurities.
 3. The method of claim 2, wherein said refined fine limecomprises less than 5% impurities.
 4. The method of claim 1, whereinsaid lime feed comprises about 94% calcium oxide and about 6%impurities.
 5. The method of claim 4, wherein said refined fine limecomprises less than 5% impurities.
 6. The method of claim 1, whereinsaid refined fine lime has a particle size distribution of greater than80% minus 200 mesh.
 7. The method of claim 1, wherein said impuritiescomprise at least one of: magnesia (MgO), silica (SiO₂), corundum(Al₂O₃), or hematite (Fe₂O₃)
 8. The method of claim 1, wherein saidmethod is performed in a low CO₂ environment.